According to the prior art, as illustrated in FIG. 1, an illumination system 10 illuminating an imager 11 is used.
Conventionally, the illumination system 10 comprises:                an illumination source 100 with an elliptical reflector;        a colour wheel 107;        a rectangular guide 102; and        a system of several relay lenses 104 to 106.        
The illumination source 100 illuminates, with a light beam 101, the colour wheel 107 placed at the entrance of the rectangular guide 102, at the focus of the elliptical reflector of the source 100. The rectangular guide 102 is used to convert the circular cross section of the illumination beam into a rectangular cross section and to make the beam spatially uniform.
The exit of the guide 102 is imaged on the imager 11 via the system of relay lenses, a minimum number of lenses being two, but often there being three or four lenses, the illumination moreover being preferably telecentric.
If the imager 11 is of the DMD type (Digital Micromirrors Device from Texas Instruments®), a TIR prism 12 is placed between the illumination system 10 and the imager 11 in order to split the illumination and imaging beams. The TIR prism 12 is unnecessary if the imager 11 is of the transmissive LCD (Liquid Crystal Display) type or is replaced with a dichroic PBS (Polarizing Beam Splitter) if the imager 11 is of the LCOS (Liquid Crystal On Silicon) type. Such a system has the drawback of limited luminosity.
To offer greater luminosity, projectors employ three imagers, each associated with one colour. However, such systems are relatively expensive.